This invention relates to impactable vehicle restraints for use at loading docks. Impactable vehicle restraints are used as a safety precaution at loading docks. Impactable vehicle restraints are used to secure a parked vehicle while the vehicle is being loaded or unloaded. Workers are commonly injured in forklift related accidents, for example, a forklift falling off the edge of a loading dock. Such accidents can occur when a trailer is not properly secured at a loading dock.
Impactable, dock face mounted impactable vehicle restraints that restrain trailers to loading docks for the safe loading and unloading of trailers have been around for over 30 years. In that time, various designs have come about including examples as found in U.S. Pat. No. 4,648,781 by Scott T. Sikora and U.S. Pat. No. 5,964,572 by Norbert Hahn and Brian Bender. These restraints are considered impactable as they are designed to be self-adjusting to the height of the rear impact guard of a trailer, otherwise called the RIG or ICC bar. When the trailer backs into the loading dock and the RIG impacts the restraint, the RIG pushes the restraint down so that it is in the proper location vertically to restrain the trailer.
One feature common to these restraint designs is the use of two switches, referred here as LS1 and LS2, which are used to determine the state of the impactable vehicle restraint. In most rotating hook impactable vehicle restraints there is a cam that operates the two switches based on the hook position, the two switches defining four zones: a hook unlocked zone, a first unsafe zone, a hook locked zone, and a second unsafe zone. LS1 is activated only in the hooked locked zone and LS2 is activated for the three zones after the hook unlocked zone. The control systems of these impactable vehicle restraints can then identify the appropriate zone based on the combined states of LS1 and LS1 and operate red and green outside and inside signal lights and an audible alarm accordingly. A drawback of this system is that from the viewpoint of the controls, both the first and second unsafe zones are the same as they have identical switch state inputs into the control system.
The most common type of impactable vehicle restraint is a rotating hook impactable vehicle restraint, which uses a rotating hook to engage the vehicle trailer. Because of the market success of rotating hook impactable vehicle restraints utilizing the two switches, the control systems that are used with them expect the two switch input as described above to function properly. In designing an impactable, vertical barrier vehicle restraint as described in U.S. Pat. No. 5,964,572 by Norbert Hahn and Brian Bender, they include an ICC sensor that is mechanically linked to the barrier to keep the sensor switches limited to two which provides compatibility with the existing control systems. In this case to keep the backward compatibility with two switch controls came with the added expense of a complex mechanical system.
During a typical process of loading and unloading a trailer, the trailer is jolted up and down due to the weight of the material handling device, typically a fork truck, driving onto and off of the trailer. With the impactable vehicle restraint self-adjusting to the height of the RIG, the restraint is jolted up and down along with the trailer during this process. This up and down motion of the impactable vehicle restraint can cause the restraining barrier to come out of a restraining position and into an unsafe one. To overcome this problem, most impactable vehicle restraint designs that use a gear motor drive system also utilize a clutch or brake in the drivetrain to retain the barrier in position to keep the vehicle restrained to the loading dock. An example of this is shown in U.S. Pat. No. 6,162,005 by Ben Fritz. A disadvantage to these systems is that they can wear out over time or require periodic maintenance to stay in safe working order.
An alternate solution to the problem of the barrier of an impactable vehicle restraint moving to an unsafe condition due to up and down trailer motion is found in U.S. Pat. No. 4,692,755 by Norbert Hahn. This patent describes energizing the motor in predefined increments if the barrier slips out of the hook locked zone to move the barrier back into the zone. The time interval to run the motor as described is 1 second, which is more time than what is needed to move the barrier from a retracted position to an raised position. An unfortunate side effect of this is that the barrier motion is stopped upon contact with either the RIG or mechanical stops in the restraint while the motor is still running. This sudden stop of the barrier combined with the continued powering of the motor results in high stresses in the operating mechanism of the impactable vehicle restraint. Over time this can cause cumulative damage in the operating mechanism components to the point of failure.